1. Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging and, particularly, to a method for three-dimensional turbo spin echo imaging.
2. Description of the Prior Art
The principles of magnetic resonance imaging (MRI) are as follows. After having applied an external magnetic field, protons in the examined tissues are excited by radio frequency (RF) pulses and they absorb certain energy, which results in their resonance. When the radiation of RF pulses is stopped, the excited protons release the absorbed energy gradually in the form of scan signals. By then acquiring the scan signals and processing them using known image reconstruction technology, a scan image of the examined tissue can be obtained. In three-dimensional MRI imaging technology, slabs are used as units for the protons in the examined tissues to be excited, with each slab including several slices.
In this case, the external magnetic field includes a main magnetic field and three orthogonal gradient magnetic fields, and in the three orthogonal gradient magnetic fields, the direction which is the same as that of the main magnetic field is usually defined as Z axis direction, with the X axis and the Y axis being orthogonal with Z axis. Specifically, the gradient magnetic field along Z axis direction is referred to as the slice selection (SS) gradient, at the same time, Z axis direction may also be referred to as SS direction; the gradient magnetic field along Y axis direction is referred to as phase encoding (PE) gradient, at the same time, Y axis direction may also be referred to as PE direction; and the gradient magnetic field along X axis direction is referred to as frequency encoding gradient, in practical applications, the frequency encoding gradient is also referred to as readout (RO) gradient, and X axis direction may also be referred to as RO direction.
The method for three-dimensional turbo spin echo (3D-TSE) imaging is an imaging method frequently used in the three-dimensional MRI technology, and FIG. 1 is a diagram of the working principles of the 3D-TSE imaging method in the prior art. The interval time between two adjacent selective excitation pulses 101 is usually referred to as a repetition time (TR), with a TR including an acquisition window and a waiting time; a first TR and a second TR are shown in FIG. 1, and the first TR will be described in detail. As shown in FIG. 1, in the acquisition window of the first TR, first, a selective excitation pulse 101 is used to excite a current slab, then a number of non-selective refocusing pulses 102 are applied, with the angle of each refocusing pulse 102 being either the same or different. When the angle of each refocusing pulse 102 is the same, this conventional 3D-TSE imaging technology. When the angle of each refocusing pulse 102 is different, then the 3D-TSE imaging technology with this feature is usually referred to as the three-dimensional spin echo imaging technology with changeable flip angles (SPACE, Sampling Perfection with Application optimized Contrast by using different flip angle Evolutions). A phase encoding gradient is applied after each time having applied a refocusing pulse 102 (not shown in the figure), then a frequency encoding gradient is applied (not shown in the figure), and one echo acquisition is carried out during the duration of the frequency encoding gradient. Thus the acquisition of scan signals and a number of echo acquisitions can form an echo chain for the subsequent reconstruction of images. During the waiting time of the first TR, the excited protons gradually return to the state before the excitation; when the waiting time ends, the excited protons have already returned to the state before the excitation, which means that the scan of the first TR has completed. In the next several consecutive TRs, the scan of the current slab can be performed repeatedly, for example, during the second TR, the above-mentioned process can be repeated, then after the completion of the scan of the second TR, the next slab is scanned, and the scan method for the next slab is the same as that of the current slab. Furthermore, the reason for the selective excitation pulse 101 being able to achieve the excitation of different slabs is that the selective excitation pulses 101 are different for different slabs, and corresponding explanation and illustration will be set forth hereinbelow.
In FIG. 1, the shape of a peak denotes the selective excitation pulse 101, and rectangles denote the non-selective refocusing pulses 102, with the height of the peak shape and rectangle denoting the size of the pulse angle. Those skilled in the art understand that the selective excitation pulse described herein refers to both the excitation pulse and slice selection gradient which are applied to the tissues to be examined simultaneously, thus making the excitation pulse selective. Such an excitation pulse with selectivity is referred to as a selective excitation pulse in the present invention. The reason for each selective excitation pulse being able to achieve the selection of different slabs is that the strength of the slice selection gradient and/or the center frequency of the excitation pulse are modulated for different slabs. The non-selective refocusing pulses described herein refer to the slice selection gradient not being applied at the same time as applying refocusing pulses to the examined tissues, thus making the refocusing pulses excite the protons in the whole examined tissue, therefore the refocusing pulses do not possess selectivity. Furthermore, for the sake of convenience, the pulses described in the present document refer to the radio frequency pulses.
It can be seen in the 3D-TSE imaging method in the prior art that only one slab can be scanned within one TR, and in order to scan the next slab, there must be a wait at least until the next TR, and the length of waiting time within a TR is far longer than the length of the acquisition window. Therefore, the imaging efficiency is reduced.
A method for multi-slice magnetic resonance imaging is disclosed in Chinese patent application no. 98121433.9. According to this method, a sequence of pulses is applied successively to different slices at M (≧2) different positions so as to obtain H (≧1) groups of magnetic resonance data, with said successively applied steps being repeated within the repetition time TR, and this method comprising the following steps that: a refocusing pulse with the selectivity only to a current slice is applied after each sequence of pulses has been applied to each slice in the repetition time TR; and then a forced recovery pulse with selectivity only to the current slice is applied.